Vim allows the use of an option call 'balloonexpr', which basically calls a function and displays the return value in a tooltip whenever the mouse cursor is at rest over a piece of text for a short period. Thus, we have:

Now, if the mouse is hovering on a fold, this will show the contents of the fold in the tooltip. If the number of folded lines is too big, it will display the first and last 15 lines only, with a comment in between indicating the number of lines that it ate.

If, however, the cursor is on a mispelled word (only works if 'spell' is set), then the tooltip will instead display a small set of suggestions for correction.